Food packaging processes of today (with the term “food” is meant all sorts of solid and liquid food, such as juices, milk and other beverages as well as pastes, soups, jellies and cheese) often are of the type “form-fill-seal” and may be carried out by shaping a continuously moving web-shaped packaging material made of a flexible laminate into a continuously running tube, continuously filling the tube with the desired food product to be packaged and by sealing and finally cutting off sealed packages from the tube. An example of such forming of a tube from a continuous web of packaging material and the further formation of packaging containers is schematically shown in FIG. 1a. 
The packaging processes are often high speed continuous processes, wherein the packaging material in the form of a web 10 is continuously fed through a machine, sterilised, for example by passing through a liquid or gas-phase quick acting sterilising medium, formed and sealed into the required tube-shape 11 for being filled with the food to be packaged and finally transversally sealed.
The continuous web-shaped packaging material is manufactured with a packaging material manufacturing machine and placed on a reel 1. The packaging material often has a laminated structure comprising a core layer of paper or paperboard, an outer heat-sealing layer of a thermoplastic polymer (such as for example polyethylene) on each side of the core layer and, if necessary, an aluminium foil gas-barrier layer interposed between the paper core layer and the film. Alternatively, a gas-barrier layer of a plastics or inorganic material, such as for example polyamide, polyethylene vinyl alcohol (EVOH) or silicon oxide, may be employed instead of aluminium foil.
The reel 1 with packaging material is installed in the packaging machine where it is reeled out and routed within the packaging machine using drive mechanisms disposed in several positions in the machine. The packaging material web is shaped into a tube and sealed in the longitudinal direction within the packaging machine. While the tube is being transferred downward within the packaging machine, the liquid or flowing food product is supplied from above to fill the inside of the tubular packaging material. Next, the packaging material tube is squeezed laterally from both sides and sealed in the lateral direction at specified intervals to form interconnected, filled and sealed packaging containers 11′. Next, the sealed packaging containers are separated off from the tube by cutting between the laterally extending sealed portions, and the thus separated packaging containers are brought into a specified, desired shape, for example by folding and bending along previously formed crease lines in the packaging material, and, if required, finally sealed in order to remain in that shape.
The sealing of the tubular packaging material in the longitudinal or lateral direction is carried out by heat sealing of the outer surfaces of the packaging material, which are made of heat sealing thermoplastics, to each other. This may be performed by known heat sealing techniques, such as for example induction heat sealing, radio frequency (RF) or microwave heat sealing, heat convection sealing or ultra-sonic vibration heat sealing. A very common heat sealing technique today for the transversal heat seals in the case of aseptic packaging, is the induction heat sealing, wherein the aluminium foil in the packaging laminate co-acts with an inductor in order to generate heat. The thermoplastic surfaces are bonded to each other by heat fusion by simultaneous application of the induction current and pressure.
Pre-cut blanks of packaging material may be fed into a packaging machine, folded and longitudinally sealed, fold-formed and sealed at the bottom in order to provide open package capsules. The capsules are filled and subsequently sealed at the top, thus providing filled packages (11″).
Conventional packaging machines thus employ a heat-sealing apparatus to seal the packaging material. The sealing apparatus is normally provided with so-called counter jaws and heat seal jaws disposed and working in opposing relation to each other.
An example of such a heat sealing apparatus is schematically shown in FIG. 4, while a cross-section view of a typical counter jaw 20 and a heat-seal jaw 30 is schematically shown in FIG. 5.
Conventionally, for transversal heat sealing, each counter jaw is provided with a pair of counter rails 21, while each heat-seal jaw is provided with a sealing block 31. Each counter rail 21 and oppositely facing seal block 31 are capable of creating one transversal heat seal S across the packaging material. A cutter 40 may be disposed in the gap formed between the two counter rails 21. Each counter rail 21 is provided with a counter element 22, a so-called “dolly” or “pressure pad”, that extends along the counter rail, while the sealing block 31, in the case of induction sealing, is provided with an inductor coil 33 disposed opposite to the dolly 22. Most commonly, in the case of packaging into a continuous tubular packaging material, the sealing and cutting operations are performed in the same part of the packaging process.
However, it is also fully possible to separate the sealing and the cutting operations from each other, for example by subsequently cutting the filled and sealed tube in a separate cutting unit.
FIGS. 2 and 3, schematically illustrate a side-view of a conventional counter rail and sealing block for induction sealing, disposed on opposite sides of the packaging material to be heat sealed, before and after the sealing S has been carried out.
As shown in FIGS. 2 and 3, the packaging material walls 12, 13 of a tube or capsule may be placed in face-to-face relation to each other in a sealing zone S, for transversal induction sealing of the for example tubular packaging material 11. Each of the packaging material walls 12, 13 is normally of a laminate structure made up of a paper base layer 14, and a film layer 16 of polyethylene, for example, located on the inside surface of the aluminium foil layer 15. Although not specifically illustrated, the outside surface of the paper base layer 14 is also coated with a layer of plastics material such as polyethylene. The polyethylene portions 16 of the two packaging materials 12, 13 are bonded together by heat fusion.
In other heat-sealing methods, such as in high frequency (RF) sealing or heat convection sealing, an aluminium foil layer is not needed for the generation of heat.
The counter rail 21 normally is made of steel, and fulfils the requirements on planarity and parallelism. Depending on i.a. the requirements of the seal quality, the type of packaging material, the size of the package and the type of product to be packed, the shape and mechanical properties of the dolly 22 may be varied to suit the circumstances best. In the case of high quality seals as in the case of the present invention, such as for example for aseptic or long-term storage, so-called “extended shelf-life” packaging, the dolly needs to have some degree of flexibility and compressibility for control of the flow of the heated thermoplastics from the layer 16 in the seal zone S.
The inductor coil 33 extends along the sealing block 31 and is normally provided with a projection 35 extending toward the counter jaws. A coolant passage 36 is formed through the inductor coil 33 to control the temperature of the inductor coil 33 as a result of coolant flowing through the coolant passage 36. In the initial stages of the sealing process shown in FIG. 2, the packaging material 11 is placed between the sealing block 31 and the counter rail 21 with dolly 22, whereupon the counter jaw and the heat seal jaw are moved so that they approach each other. Subsequently, the counter jaw and the heat seal jaw are moved further towards each other, and the sealing portion of the packaging material 11 is pressed hard and deformed with the inductor coil 33 and the counter element 22. A high frequency voltage is applied with a power device (not shown) to cause the aluminium foil 15 to generate heat with induction current. As a result, as shown in FIG. 3, the paired polyethylene portions 16 of the packaging material facing each other and squeezed between the paired aluminium foils 15 are heated and the polyethylene portions 16 in the sealing zone S are fused.
Consequently, the tubular packaging material 11 is bonded together by heat fusion.
As shown in FIG. 3, the compressible counter element 22 is deformed during the sealing stage. When the pressure from the sealing block and the counter rail is released, the counter element is resuming its original shape and is ready for the next sealing and compression operation. Such compressible counter elements are conventionally made of a plastics material with suitable mechanical and chemical properties. Today, most commonly a cross-linked polyurethane (PUR) is used for this purpose. The desired shape and configuration of the dolly is usually cut out from a cross-linked polyurethane material. The dolly of cross-linked PUR is fastened into the cutting rail of stainless, chemically resistant steel by insertion into a groove 27 extending along the counter rail 21. The configuration, hardness and compressibility of the dolly are factors of great importance to the quality of the seal, and may vary depending on the various factors listed above, i.e. required seal quality, type of packaging material, package size and product to be filled. Different shapes and hardness/compressibility properties of the dolly will influence the flow of thermoplastics in the seal zone S during heat fusion differently. Furthermore, these are important factors influencing the way in which the filled product in the tube is squeezed away from the sealing zone S. Different shapes have thus proved to be optimal for different combinations of package sizes and products to be filled.
Furthermore, the plastics material used in the dolly should be resistant to chemicals (for example alkaline cleaning agents, lactic acid and other substances in various filling products and to sterilisation agents, such as for example hydrogen peroxide (H202).
Although the known counter rail and dolly functions quite adequately, it does have a number of disadvantages. The main drawback with this known construction is that the dolly is made of a rather soft material in relation to the cutting rail and the sealing block, and will wear out after some time and thus must be exchanged for a new one with regular intervals. Each time the dolly is changed the packaging machine has to be stopped entirely. First, the counter rail has to be removed from the counter jaw, to which it is attached during operation. Then, the old dolly, which is fastened into the groove in the counter rail, has to be removed.
When the old dolly has been removed, a new fresh dolly must be inserted into the narrow groove of the counter rail and subsequently the counter rail has to be attached to the counter jaw and the machine started up again. The groove, as well as the dolly, usually has an asymmetrical cross-section configuration and it is important that the dolly is carefully fastened and secured into the groove and that it is inserted in the right position, i.e. oriented in the right way. The steps of changing the dolly take some time, since the dolly should be quite strongly fastened into the groove and the dolly, therefore, has a slightly larger cross-section than the groove. Accordingly, the dolly must be pulled out of, respectively pressed into, the groove by using some force. This is done manually, since it is a quite complicated operation. The time the machine has to be completely stopped may amount to up to about 10 minutes, including the slow-down and start-up time during which the machine is adjusted from/to normal operation speed, during which time at least 600-1300 packages could have been produced at normal production speed, depending on the type of packaging machine.
A device for heat-sealing a tube of sheet packaging material filled with a pourable food product is described in EP 1 300 340 A1. The device has a first and a second jaw having sealing means and pressure means, respectively. The jaws are movable towards each other in a direction transverse to the feed path of the tube in order to grip the tube at a certain portion of the tube and to seal the tube at said portion. The first jaw (usually named sealing jaw) defines a first contact surface cooperating with the tube of packaging material and has at least one projection; and the second jaw (usually named counter jaw) defines a second contact surface, which is convex at least at the projection of the first contact surface.
A similar device for heat sealing is disclosed in U.S. Pat. No. 6,216,420 B1. The device for heat sealing a tube of packaging sheet material has first and second jaws movable towards each other to grip the tube under pressure and heat seal the tube. The first jaw has a heating element having a pair of substantially straight active surfaces, which extend on opposite sides of and parallel to an intermediate plane. The second jaw has a pair of pressure pads, which are housed in respective seats and cooperate with the active surfaces of the heating element.
WO 00/44625 relates to a counter rail and dolly, suitable for use in an apparatus for heat sealing a laminated packaging material for packaging of a liquid or a flowing product, wherein the counter rail and the dolly each are made of a substantially plastics material. The document also relates to an apparatus for heat sealing and to a sealing/cutting apparatus including said counter rail and dolly. Furthermore, the document discloses a method of producing said counter rail and dolly by means of co-injection moulding.
When designing the above-described heat-sealing apparatuses a critical factor is the lifetime of the rubber part forming the contact surface of the counter jaw. This part of the heat sealing apparatus is in commercial applications of today often replaced at intervals of less than about 40 hours of production due to the occurrence of unwanted marks or cuts on the abutment surface.
Moreover, a problem often arising is that the rubber dolly has a tendency to stick to the packaging material which might cause forming problems and damages to the packaging material and the to the rubber dolly. Another problem with the sealing system according to the conventional technology is that the sealing window, i.e. the interval in between the essential sealing parameters can be chosen, is to narrow to give the machine operators a satisfactory safety margin between optimal seals and non satisfactory seals.